Switched-mode power converters are widely used for efficiently converting an input direct-current (DC) voltage into an output power supply at a different voltage level. Switched-mode power converters are often used as alternatives to linear regulators, but have the advantage of being able to step a voltage up or down. Additionally, switched-mode power converters are significantly more efficient than linear regulators, as linear regulators waste considerable energy in ohmic losses. Because wasted energy must typically be dissipated as heat, switched-mode power converters have reduced heat dissipation requirements, relative to other power converters such as linear regulators, meaning that fewer and/or smaller heat sinks are required. For these reasons, switched-mode power converters are commonly used for converting DC voltage levels in applications requiring high efficiency.
Isolated switched-mode power converters use a transformer to provide galvanic isolation between a primary (input) and secondary (output) side of the converter. Switch devices on the primary side of the transformer are used to create an alternating current (AC) voltage component from an input DC voltage. This AC voltage is used to energize a primary winding of the transformer which, in turn, induces an AC voltage on a secondary winding of the transformer. The AC voltage on the secondary winding is rectified and filtered to provide a DC output voltage that may be used by some output load of the power converter.
The DC output voltage and current is determined by the switching of the primary side switch devices. In typical applications, a pulse-width modulated (PWM) waveform is used to control each of the primary side switch devices such that they provide an appropriate level of energy to the primary winding which, ultimately, determines the voltage output from the power converter. A secondary side controller typically monitors the output voltage and current, and generates appropriate PWM waveforms which are transferred to the primary side for use by the primary side switch devices. The secondary side controller may perform other functions including, e.g., providing control signals to rectify the secondary side AC voltage using switch devices on the secondary side.
The secondary side controller is, typically, a digital circuit that must be powered during both start-up and fully operational phases of the power converter. Current solutions for providing power to a secondary side controller usually employ an auxiliary power supply on the primary side which is powered by the DC input voltage. A second transformer is included in the power converter and transfers power from this primary side auxiliary supply to an auxiliary supply on the secondary side so that the secondary side auxiliary supply can provide power to the secondary side controller. Such solutions have the disadvantages that they require a second transformer (or an additional pair of windings within an existing transformer) as well as an auxiliary power supply on the primary side.
Circuits and methods for providing an auxiliary power supply on a secondary side of an isolated switched-mode power converter are desired. Such circuits and methods should require minimal circuit components beyond what is required for the power converter, and should be capable of supplying power to a secondary side controller during start-up and steady-state operation of the power converter.